Bi-amping with Avatar Cabs and QSC PLX

I know bi-amping has been discussed a lot in here, and I did search a bunch of threads without finding what I'm looking for.

I currently have one Avatar 4x10 cab and I'm looking into adding something for low end. Now I really like the range of the 4x10, however it starts breaking up on the lows whenever I try to play a 5 string through it.

So I want to add either a 1x15 or a 2x12 to even out the low end. Do you think I need to bi-amp it? It makes sense that it'd be best to cut out some of the lows on the 4x10 and pass them to the 2x12 or 1x15, but I don't want to cut out the highs from those speakers also.

It seems what I really would just need is a frequency filter on the 4x10 side. Now the PLX 3402 I have powering it does have a low pass filter at either 50 or 30 Hz, however I don't think it can do it for individual channels. There's this thing: http://www.qscstore.com/uf3.html which looks like it may be able to do it (Help me out Bob Lee ).

Overall I'm getting a bit confused and could use some suggestions to make this rig work out. I'd prefer to get the 2x12 over the 1x15 because it would be more versatile if I had to use it by itself.

It can! I'm actually looking at getting an avatar 2x10 and 2x12 and doing the same setup. If you run one cabinet per side of the amp you can filter out those frequencies. You can do this with the switches on the back.

The low-freq filters on a PLX amp are separate for each channel. I would recommend engaging the filter on the channel driving the speaker cabinet you're having trouble with. Experiment. If 30 Hz works, great. If not, use 50 Hz; but be aware it'll roll off more of the low end.